Engineered polypeptides are used widely in therapy and diagnostic applications. Therapeutic antibodies have been useful for many years in, for example, treatment of cancer and inflammatory conditions. Therapeutic polypeptides are also used to treat and prevent blood conditions and viral infections. Diagnostic polypeptides have been used successfully to identify healthy and diseased cells and tissues in vivo.
Many polypeptides can provide targeting functionality to specific cells. The selective affinity of certain polypeptides can be used to target nearly any cell or tissue desired, for example a cell expressing an antigen. A polypeptide can carry a molecular payload to slow or destroy the target cell or tissue. Polypeptides have thus found use in therapy for conditions such as cancer, inflammatory diseases, autoimmune diseases and transplant rejection.
In certain applications therapeutic polypeptides are linked to molecular shields to increase their lifetime within an organism. Polypeptides have also found use as diagnostics. These polypeptides can carry a label to indicate the presence of a target receptor on a cell or in a tissue. These labels are typically linked to the polypeptides by covalent bonds.
To date, techniques for linking polypeptides to molecular entities such as molecular payloads, including molecular shields and labels, have been limited by their heterogeneity in degree and location of linking to the polypeptides, by their low yields and by losses in activity. Typical conjugation sites include random locations on polypeptide chains, e.g. random amines on amino acid side chains, and the N-terminus of certain polypeptide chains. In such techniques, some polypeptides might be linked to the conjugate at one location while some polypeptides are linked to the same conjugate at another location, and some polypeptides might not be linked at all.
There is a need, therefore, for polypeptides modified at site-specific positions optimized for uniformity, yield and/or activity to further the promising use of polypeptides in, for example, therapy and diagnostics.